There is an increasing demand for rate gyroscopes--that is to say, gyroscopes that may be used to sense rates of turn--that contain the minimum of moving parts and are therefore potentially simpler, cheaper and more robust than the traditional rate gyroscopes that use rotors and gimbal-type suspensions. An increasing number of rate gyroscope designs have recently made use of the piezo-electric effect in order to cut down the number of moving parts and to do away with rotors. For example, published UK Patent Application No. GB 2111209A, which formed the priority basis for U.S. Pat. No. 4,489,609, describes three designs of one known type of gyroscope making use of this effect. In one of these, the piezo-electric effect is used to excite a long beam, held at both ends of its axis Ox, to deflect in one of the planes (the Oxy plane) including that axis: when the beam is subjected to a rate of turn about Ox, the vibration in the Oxy plane gives rise, by reason of the Coriolis, effect to inertia forces that cause it to vibrate in the Oxz plane, and these vibrations are sensed to give a measure of the rate of turn. In the second example, a mushroom-shaped structure comprising a stem and surmounting head formed about axis Oz is excited into a mode of resonant vibration in which the head expands and contracts radially, that is to say in the Oxy plane: when the device is subjected to a rate of turn about Oz, the Coriolis effect results in resonant torsional vibrations of the stem about Oz from which a measure of the applied rate may again be derived. In the third example, the sensitive structure is a rectangular plate arranged so that its edges lie parallel to the axes Ox and Oz, and its thickness dimension parallel to Oy: the plate is located by supports attached to the opposite edges that lie parallel to Oz, and is excited so as to execute resonant vibrations in a direction parallel to Oz, and when the plate is subjected to a rate of turn about Oy the Coriolis effect tends to generate vibrations parallel to Ox, giving rise, because of the piezo-electric effect, to signals from which a measure of the applied rate of turn may once again be derived.
While all three of the constructions just described have the advantage that they lack conventional moving parts, so that the movements essential to the gyroscopic effect are only vibratory movements of anchored structures, nevertheless in each case the vibrations that need to be sensed in order to indicate the applied rate of turn are significantly different from those into which the structure is originally excited. For instance they are directed along a different axis, or are generated in a different part of the total structure.
UK Specification No. GB 2061502A shows an example of another known type of gyroscope which makes use of the piezo-electric effect. The gyroscope described in that specification comprises an accurately-machined cup-shaped resonator make of some ordinary and suitably robust material, to the wall of which excitation and detection piezo-electric vibration transducers are bonded. An input current to the excitation transducers produces a mechanical output which in turn sets the resonator into vibration, and the variation of that vibration due to an applied rate of turn is sensed by the detection transducers and provides an indication of the magnitude of that rate of turn.
Not only do the meeting faces of such resonators and piezo-electric transducers have to be of accurately-matching shape in order for it to be possible to make a satisfactory bond between them, but also the complex of vibrations which such a resonator undergoes in use subjects the bonds to great strain. While the failure of a bond of course results in the failure of the whole instrument, mere deterioration of a bond short of total failure will result in inaccuracy of the gyroscope which may not at once be apparent.